Boiler and method for controlling boiler

ABSTRACT

An object of the present disclosure is to appropriately reduce NOx and CO. A boiler includes: a can body having water pipe; a burner for supplying primary fuel and air into the can body; a secondary fuel supply unit for supplying secondary fuel into the can body downstream of the burner in a flow direction of combustion gas; a cooling line for introducing a cooling fluid for reducing temperature of a predetermined space in the can body downstream of the burner in the flow direction of the combustion gas; a flow rate adjusting unit capable of adjusting a flow rate of the cooling fluid introduced into the can body from the cooling line; and a control unit for controlling the flow rate adjusting unit to control the flow rate of the cooling fluid such that the temperature of the predetermined space is 800° C. or more and 1200° C. or less.

BACKGROUND 1. Technical Field

The present disclosure relates to a boiler and a method for controllingthe boiler.

2. Description of the Related Art

As the boiler for generating steam using heat obtained by burning a fuelgas, as disclosed in, for example, JP-A-2006-220373 andJP-A-2011-133180, there is a boiler which performs two-stage combustionby further including a fuel supply unit for supplying the fuel gas to adownstream side of a burner for supplying the fuel gas into a can body.JP-A-2006-220373 and JP-A-2011-133180 describe that NOx, CO, and oxygenconcentrations contained in exhaust gas can be reduced with the aboveconfiguration. Further, JP-A-2011-133180 describes that the exhaust gasfrom the can body is drawn by an ejector and recirculated to the canbody.

SUMMARY

For example, in JP-A-2011-133180, a position to which combustion gas issupplied is adjusted depending on an amount of combustion; however,there is room for further improvement in controlling temperatureappropriately in the can body to reduce NOx and CO in the two-stagecombustion described in JP-A-2006-220373 and JP-A-2011-133180.

An aspect of the present disclosure aims to provide the boiler forappropriately reducing NOx and CO and the method for controlling theboiler

According to an aspect of the present disclosure, there is provided aboiler including: a can body having water pipe; a burner connected tothe can body and for supplying primary fuel and air into the can body; asecondary fuel supply unit for supplying secondary fuel into the canbody downstream of the burner in a flow direction of combustion gas; acooling line for introducing a cooling fluid for reducing temperature ofa predetermined space in the can body downstream of the burner in theflow direction of the combustion gas; a flow rate adjusting unitprovided in the cooling line and capable of adjusting a flow rate of thecooling fluid introduced into the can body from the cooling line; and acontrol unit for controlling the flow rate adjusting unit to control theflow rate of the cooling fluid introduced into the can body such thatthe temperature of the predetermined space is 800° C. or more and 1200°C. or less.

According to an aspect of the present disclosure, there is provided amethod for controlling a boiler including: a can body having water pipe;a burner connected to the can body and for supplying primary fuel andair into the can body; a secondary fuel supply unit for supplyingsecondary fuel into the can body downstream of the burner in a flowdirection of combustion gas; a cooling line for introducing a coolingfluid for reducing temperature of a predetermined space in the can bodydownstream of the burner in the flow direction of the combustion gas;and a flow rate adjusting unit provided in the cooling line and capableof adjusting a flow rate of the cooling fluid introduced into the canbody from the cooling line. The method controls the flow rate adjustingunit to control the flow rate of the cooling fluid introduced into thecan body such that the temperature of the predetermined space is 800° C.or more and 1200° C. or less.

According to an aspect of the present disclosure, there is provided theboiler for appropriately reducing NOx and CO and the method forcontrolling the boiler.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of a boiler according to afirst embodiment;

FIG. 2 is a schematic partial cross-sectional view of the boileraccording to the first embodiment;

FIG. 3 is a schematic block diagram of a control device according to thefirst embodiment;

FIG. 4 is a graph showing a relationship between an amount of primaryfuel, an amount of secondary fuel, and an amount of cooling fluid;

FIG. 5 is a flowchart showing an example of a control method of theboiler according to the present embodiment;

FIG. 6 is a schematic cross-sectional view of the boiler according to asecond embodiment; and

FIG. 7 is a schematic view of an ejector according to the secondembodiment.

DESCRIPTION OF THE EMBODIMENTS

Hereinafter, embodiments of the present disclosure will be describedwith reference to the drawings, but the present disclosure is notlimited thereto. Components of the embodiments described below can becombined as appropriate. In addition, some components may not be used.

In the following description, combustion gas is a concept including atleast one of gas in which combustion reaction of fuel gas is completedand the fuel gas during the combustion reaction, and including all of acase where both the gas in which the combustion reaction of the fuel gasis completed and the fuel gas during the combustion reaction areincluded, a case where only the fuel gas during the combustion reactionis included, and a case where only the gas in which the combustionreaction of the fuel gas is completed is included.

First Embodiment (Entire Structure of Boiler)

FIG. 1 is a schematic cross-sectional view of a boiler according to afirst embodiment. FIG. 2 is a schematic partial cross-sectional view ofthe boiler according to the first embodiment. FIG. 2 is a schematicpartial cross-sectional view when a boiler 1 according to the firstembodiment is viewed from a Z direction described below, that is, whenthe boiler 1 is viewed from upward in a vertical direction. As shown inFIGS. 1 and 2, the boiler 1 according to the first embodiment has a canbody 10, a blower 12, a duct 14, a burner 16, an exhaust stack 18, afuel supply unit 20 (see FIG. 2), a primary fuel supply unit 22, asecondary fuel supply unit 24 (see FIG. 2), a cooling line 26 (see FIG.2), a flow rate adjusting unit 28 (see FIG. 2), and a control device 30.

As shown in FIG. 1, the can body 10 has a main body 40, an upper header42, a lower header 44, and a water tube group 50. In the presentembodiment, an X direction is a flow direction of the combustion gasflowing in the main body 40. Further, a direction perpendicular to the Xdirection and in a vertical direction, which is upward in the verticaldirection, is a Z direction. The Z direction is a direction being thesame as the vertical direction in the present embodiment, but is notlimited to the vertical direction. The main body 40 is a housing havinga longitudinal direction in the X direction, and accommodates the watertube group 50 therein. The upper header 42 is a header connected to anend portion in the Z direction of the main body 40, here, the endportion in the vertical direction of the main body 40. The lower header44 is a header connected to an end portion in a direction opposite tothe Z direction of the main body 40, here, the end portion downward inthe vertical direction of the main body 40.

The water tube group 50 has a plurality of water tubes 51, 52, 53 inwhich water or steam flows. The water pipes 51, 52, 53 are provided inthe main body 40 and extend in the Z direction, and connect the upperheader 42 and the lower header 44. As shown in FIG. 2, a plurality ofwater pipes 51 (outside water pipes) are located at both ends of themain body 40 and are aligned in a direction of the combustion gasflowing, that is, in the X direction. A plurality of water pipes 53(central water pipes) are located inside the water pipes 51 and arealigned in the X direction. A plurality of water pipes 52 (intermediatewater pipes) are located between the water pipes 51 and the water pipes53, and are aligned in the X direction. Further, in the main body 40, aconnecting wall 54 connecting the plurality of water tubes 51 isprovided extending in the X direction. A space surrounded by the waterpipes 51, the connecting wall 54, the upper header 42, and the lowerheader 44 forms a gas flow space. The water pipes 52 are arranged suchthat a distance between a part of the water pipes 52 in the X directionis longer than the distance between the other water pipes 52 in the Xdirection. That is, the water pipes 52 are partially removed. In the gasflow space, the space between the water pipes 52 having this longdistance, that is, the space in which the pipe is removed is wider thanthe space between the other water pipes. Hereinafter, the space betweenthe water pipes 52 having the long distance will be referred to as acombustion promoting space S. Since the combustion promoting space S iskept wide, combustion, that is, oxidation of CO in the combustion gas ispromoted. The combustion promoting space S is not limited to the spacebetween the water pipes 52, as long as it is a space kept wider than theother spaces surrounded by the water pipes, and may be a spacesurrounded by any water pipes. For example, in the main body portion 40,a diameter of the water pipe in the space upstream in the flow directionof the combustion gas is made smaller than that of the water pipes inthe space downstream in the flow direction of the combustion gas, sothat a pitch of the water pipes in the space upstream in the flowdirection of the combustion gas may be larger than that of the waterpipes in the space downstream in the flow direction of the combustiongas. In this case, the combustion promoting space S may be provided inthe space upstream in the flow direction of the combustion gas. However,the combustion promoting space S may not necessarily be provided.

The blower 12 supplies air A for mixing with a primary fuel F1 describedbelow. The duct 14 is a duct connected to the blower 12 and the air Asupplied from the blower 12 flows therethrough. The primary fuel supplyunit 22 is connected to the duct 14. Specifically, as shown in FIG. 2,the primary fuel supply unit 22 has a fuel supply line 60 and a primaryfuel adjusting valve 62. One end of the fuel supply line 60 is connectedto the fuel supply unit 20 for supplying the fuel F, and the other endthereof is connected to the duct 14. The fuel supply line 60 is providedwith the primary fuel adjusting valve 62. The fuel F supplied from thefuel supply unit 20 flows through the fuel supply line 60, and at leasta part of the flowing fuel F is supplied into the duct 14 as the primaryfuel F1 through the primary fuel adjusting valve 62. The primary fuel F1supplied from the fuel supply line 60 is mixed with the air A suppliedfrom the blower 12 in the duct 14 to generate a mixed gas F1A. The fuelF (that is, the primary fuel F1 and a secondary fuel F2 described below)is a combustible fuel gas, such as natural gas and propane gas, but maybe any fuel. Further, the primary fuel adjusting valve 62 is controlledto be opened and closed by the control device 30, to adjust a supplyamount of primary fuel F1 to the duct 14.

Further, as shown in FIG. 1, the duct 14 has a pressure reducing member12A upstream of a portion to which the fuel supply line 60 is connected,in flow of the air A. The pressure reducing member 12A is a member forreducing a pressure of the air A downstream thereof in the flow of theair A than the pressure of the air A upstream thereof in the flow of theair A, for example, by throttling a flow path. The pressure reducingmember 12A is, for example, a punching metal in the present embodiment.The boiler 1 has an air differential pressure sensor 12B for detecting adifferential pressure between the pressure downstream of the pressurereducing member 12A and the pressure upstream of the pressure reducingmember 12A. The air differential pressure sensor 12B detects thedifferential pressure between the pressure of the air A downstream ofthe pressure reducing member 12A and the pressure of the air A upstreamof the pressure reducing member 12A in the duct 14, and outputsinformation on the detected differential pressure to the control device30.

As shown in FIG. 2, the primary fuel supply unit 22 further includes apressure reducing member 64 and a fuel differential pressure sensor 66.The pressure reducing member 64 is a member provided in the fuel supplyline 60 and for reducing a pressure of the primary fuel F1 downstreamthereof in flow of the fuel F than the pressure of the primary fuel F1upstream thereof in the flow of the fuel F, for example, by throttling aflow path. More specifically, the pressure reducing member 64 isprovided downstream of the primary fuel adjusting valve 62 and upstreamof a connection portion of the fuel supply line 60 with the duct 14. Inthe present embodiment, the pressure reducing member 64 is, for example,an orifice. Further, the fuel differential pressure sensor 66 detects adifferential pressure between the pressure downstream of the pressurereducing member 64 and the pressure upstream of the pressure reducingmember 64. The fuel differential pressure sensor 66 detects adifferential pressure between the pressure of the primary fuel F1downstream of the pressure reducing member 64 and the pressure of theprimary fuel F1 upstream of the pressure reducing member 64 in the fuelsupply line 60, and outputs information on the detected differentialpressure to the control device 30.

The duct 14 is also connected to the main body 40 of the can body 10.The duct 14 is connected to a portion opposite to the X direction of themain body 40, that is, an upstream portion in the flow direction of thecombustion gas. Further, a burner 16 is provided at a connection portionbetween the duct 14 and the main body 40. That is, it can be said thatthe burner 16 is connected to the can body 10 and is connected to aportion opposite to the X direction of the main body 40. The mixed gasF1A flowing through the duct 14 is supplied to the burner 16. The burner16 supplies the mixed gas F1A, that is, the primary fuel F1 and the airA into the main body 40 of the can body 10.

The exhaust stack 18 is connected to the main body 40 of the can body 10and, more specifically, connected to a portion in the X direction of themain body 40 (the most downstream portion in the flow direction of thecombustion gas). The combustion gas in the main body 40 is discharged asexhaust gas from inside the main body 40 to the exhaust stack 18.

The secondary fuel supply unit 24 supplies the secondary fuel F2 intothe can body 10 downstream of the burner 16 in the flow direction of thecombustion gas. Specifically, as shown in FIG. 2, the secondary fuelsupply unit 24 has secondary fuel supply lines 70, 74 and a secondaryfuel adjusting valve 72. The secondary fuel supply line 70 is connectedto the fuel supply line 60. More specifically, the secondary fuel supplyline 70 is connected to the fuel supply line 60 at a portion between theconnection point with the fuel supply unit 20 and the connection pointwith the primary fuel adjusting valve 62. Accordingly, in the fuelsupply line 60, at least a part of the fuel F is supplied as thesecondary fuel F2 from the fuel supply unit 20. The secondary fueladjusting valve 72 is provided in the secondary fuel supply line 70. Thesecondary fuel adjusting valve 72 adjusts a supply amount of secondaryfuel F2 to the secondary fuel supply line 70 by the control device 30controlling opening and closing.

The secondary fuel supply line 74 is connected to a portion downstreamof the connection portion of the secondary fuel supply line 70 with thesecondary fuel adjusting valve 72 in flow of the secondary fuel F2. Thesecondary fuel supply line 74 is supplied with the secondary fuel F2from the secondary fuel supply line 70 through the secondary fueladjusting valve 72. The secondary fuel supply line 74 is also connectedto the main body 40 of the can body 10. More specifically, the secondaryfuel supply line 74 is connected to a portion in the X direction(downstream in the flow direction of the combustion gas) than aconnection portion of the main body 40 with the burner 16. Therefore,the secondary fuel supply line 74 supplies the secondary fuel F2 fromthe secondary fuel supply line 70 into the can body 10 downstream of theburner 16 in the flow direction of the combustion gas. Furthermore, thesecondary fuel supply line 74 is connected to a downstream side of aportion in which the combustion by the primary fuel F1 starts in the canbody 10, that is, the downstream side of an ignition unit (not shown).The secondary fuel supply line 74 is preferably connected to a positionin which temperature of the combustion gas is 800° C. or higher, thatis, the position in which the secondary fuel F2 is appropriatelyself-combusted to a temperature capable of suppressing CO generation.Further, the secondary fuel supply line 74 is preferably connected to aportion opposite to the X direction (upstream in the flow direction ofthe combustion gas) than the combustion promoting space S. In thepresent embodiment, two secondary fuel supply lines 74 are providedbranched from the secondary fuel supply line 70, and each of them isconnected to the main body 40. However, the number of secondary fuelsupply lines 74 is arbitrary, and may be one, for example.

As shown in FIG. 2, one end of the cooling line 26 is connected to theexhaust stack 18 and the other end thereof is connected to the secondaryfuel supply line 70. The exhaust gas from the exhaust stack 18, that is,the combustion gas discharged from the can body 10 is supplied as acooling fluid G0 to the cooling line 26. The cooling line 26 isconnected to a portion downstream of the connection portion with thesecondary fuel adjusting valve 72 and upstream of the connection portionwith the secondary fuel supply line 74 of the secondary fuel supply line70 in the flow of the secondary fuel F2. Therefore, the cooling fluid G0flowing through the cooling line 26 is supplied to the secondary fuelsupply line 74 through the secondary fuel supply line 70, and introducedinto the can body 10 downstream of the burner 16 in the flow directionof the combustion gas from the secondary fuel supply line 74.Furthermore, the secondary fuel F2 is also supplied to the secondaryfuel supply line 74. Therefore, in the secondary fuel supply line 74,the secondary fuel F2 and the cooling fluid G0 are mixed to generate amixed gas F2A. The secondary fuel supply line 74 introduces the mixedgas F2A, that is, the secondary fuel F2 and the cooling fluid G0 intothe can body 10.

The flow rate adjusting unit 28 is provided in the cooling line 26. Theflow rate adjusting unit 28 adjusts a supply amount of cooling fluid G0introduced into the can body 10 from the cooling line 26 through thesecondary fuel supply line 74 by control of the control device 30. Inthe present embodiment, the flow rate adjusting unit 28 is a fan. Theflow rate adjusting unit 28 draws the cooling fluid G0 (exhaust gas)from the exhaust stack 18 to supply it into the cooling line 26, andsupplies the cooling fluid G0 supplied into the cooling line 26 into thecan body 10 through the secondary fuel supply line 74. The flow rateadjusting unit 28 adjusts the supply amount of cooling fluid G0 to besupplied to the can body 10 by the control device 30 controllingrotational speed of built-in vane (not shown). However, the flow rateadjusting unit 28 is not limited to controlling the rotational speed,and may adjust the supply amount of cooling fluid G0 by any method, aslong as it can adjust the supply amount of cooling fluid G0 to besupplied by the control of the control device 30. For example, the flowrate adjusting unit 28 may adjust the supply amount of cooling fluid G0by controlling an opening degree of the built-in vane.

In the present embodiment, the cooling line 26, the secondary fuelsupply line 70, and the secondary fuel supply line 74 are described asseparate pipes. However, since the cooling line 26, the secondary fuelsupply line 70, and the secondary fuel supply line 74 are connected toeach other, the cooling line 26, the secondary fuel supply line 70, andthe secondary fuel supply line 74 can be rephrased as one tube.

In the boiler 1 structured as described above, first, the primary fuelF1 introduced from the fuel supply line 60 and the air A supplied fromthe blower 12 are mixed in the duct 14 to generate the mixed gas F1A.The mixed gas F1A is supplied from the burner 16 into the main body 40of the can body 10. The mixed gas F1A supplied into the main body 40 isignited by the ignition unit (not shown), and the combustion gas withflame in the combustion reaction is formed by the burner 16. Thecombustion gas flows in the X direction while exchanging heat with thewater pipes 51, 52, 53 in the main body 40. The cooling fluid G0introduced from the cooling line 26 and the secondary fuel F2 introducedfrom the secondary fuel supply line 70 are mixed in the secondary fuelsupply line 74, and introduced to a portion downstream of the burner 16in the main body 40 in the flow of the combustion gas as mixed gas F2A.The mixed gas F2A is burned in contact with the combustion gas. Thus,the boiler 1 performs two-stage combustion by supplying the mixed gasesF1A and F2A from the burner 16 and the secondary fuel supply line 74.The combustion gas supplied with the mixed gas F2A and burned in twostages further flows in the X direction while exchanging heat with thewater pipes 51, 52, 53 in the main body 40, and is discharged from theexhaust stack 18 as the exhaust gas.

The boiler 1 according to the present embodiment can reduce an amount ofNOx and an amount of CO contained in the combustion gas discharged fromthe can body 10 by performing the two-stage combustion in this manner.Here, in order to make the secondary fuel F2 self-burn to suppress thegeneration of CO, it is preferable to maintain temperature at a supplyposition of the secondary fuel F2 high to some extent. However, if thetemperature at the supply position of the secondary fuel F2 is too high,combustion temperature (maximum temperature reached by the combustion)by the secondary fuel F2 may be too high, and the amount of NOx in thecombustion gas may increase. Therefore, when performing the two-stagecombustion in the boiler 1, it is preferable to maintain the temperaturein the can body 10 within a predetermined range in order to suppress theamount of NOx and the amount of CO.

The boiler 1 according to the present embodiment maintains thetemperature in the can body 10 within the predetermined range byintroducing the cooling fluid G0 into the can body 10. The cooling fluidG0 is a fluid that is not burned by the combustion gas, and has a lowertemperature than the combustion gas in the can body 10, so that thetemperature in the can body 10 can be reduced. Here, the position atwhich the cooling fluid G0 is supplied in the can body 10, in otherwords, the position at which the secondary fuel supply line 74 isconnected in the can body 10, is taken as the supply position. Thecooling line 26 reduces the temperature of the supply position bysupplying the cooling fluid G0 to the supply position in the can body10. Here, a space in the can body 10 between the supply position and aposition in the X direction by a predetermined distance from the supplyposition is taken as a predetermined space. The cooling line 26 can besaid to reduce temperature of the predetermined space by supplying thecooling fluid G0 to the supply position. In the first embodiment, thepredetermined space is a space from the supply position to which thesecondary fuel F2 is supplied to a downstream side of the combustiongas. That is, it can be said that the cooling line 26 reduces thetemperature of the space in which an unburned portion of the primaryfuel F1, and the secondary fuel F2 or the secondary fuel F2 burn bysupplying the cooling fluid G0 to the supply position. In other words,it can be said that the predetermined space is a space in which at leastthe secondary fuel F2 out of the primary fuel F1 and the secondary fuelF2 is burned. In the present embodiment, since the supply position isfixed, the predetermined space is a space whose position is fixed in thecan body 10, and the position does not move in the can body 10.

As described above, the boiler 1 reduces the temperature of thepredetermined space in the can body 10 by introducing the cooling fluidG0 into the can body 10. Further, the boiler 1 maintains the temperaturein the can body 10 within the predetermined range by adjusting theamount of cooling fluid G0 to be supplied by the control device 30. Thecontrol device 30 will be described below.

(Structure of Control Device)

FIG. 3 is a schematic block diagram of the control device according tothe first embodiment. As shown in FIG. 3, the control device 30 has acontrol unit 80 and a storage unit 82. The control device 30 is acomputer for controlling the boiler 1. The storage unit 82 is a memoryfor storing operation contents of the control unit 80, information onprogram, and the like. The storage unit 82 includes, for example, atleast one external storage device such as a RAM (Random Access Memory),a ROM (Read Only Memory), or a flash memory.

The control unit 80 is a computing device, that is, a CPU (CentralProcessing Unit). The control unit 80 has an air controller 84, aprimary fuel controller 86, a secondary fuel controller 88, and a fluidcontroller 90. The air controller 84, the primary fuel controller 86,the secondary fuel controller 88, and the fluid controller 90 performprocessing to be described below by reading software (program) stored inthe storage unit 82. However, the air controller 84, the primary fuelcontroller 86, the secondary fuel controller 88, and the fluidcontroller 90 may be respectively constituted by dedicated hardwarecircuits.

The air controller 84 calculates the supply amount of air A supplied tothe can body 10 and controls the supply amount of air A supplied to thecan body 10 so as to be the calculated supply amount. Specifically, theair controller 84 calculates the supply amount of air A depending on,for example, a combustion stage instructed to the boiler 1, that is, aninstruction of what kind of combustion stage the boiler 1 is operatedin. For example, in the present embodiment, information indicating arelationship between the combustion stage and the supply amount of air Ais stored in the storage unit 82. The air controller 84 reads thisinformation from the storage unit 82 and substitutes the instructedcombustion stage into the relationship to calculate the supply amount ofair A. For example, the supply amount of air A is set to be larger asthe instructed combustion stage is larger, that is, as the combustion ishigher. A method for calculating the supply amount of air A is notlimited to this, and may be set arbitrarily. Further, in the boiler 1 inthe present embodiment, a plurality of combustion stages is set for eachamount of combustion, and for example, four of stop, low combustion,medium combustion, and high combustion are set.

The air controller 84 controls the blower 12 so that the amount of air Athus calculated is supplied. For example, the air controller 84 suppliesthe calculated amount of air A to the duct 14 by adjusting an openingdegree of a damper (not shown) provided upstream of the pressurereducing member 12A shown in FIG. 1. For example, the air controller 84obtains information on the differential pressure between the upstreamside and the downstream side of the pressure reducing member 12A fromthe air differential pressure sensor 12B shown in FIG. 1. The aircontroller 84 obtains the amount of air A actually supplied to the duct14 from the information on the differential pressure detected by the airdifferential pressure sensor 12B, and adjusts the opening degree of thedamper to actually supply the calculated amount of air A based on theobtained actual supply amount of air A. The air controller 84 is notlimited to adjusting the opening degree of the damper when controllingthe supply amount of air A. For example, the air controller 84 maycontrol the supply amount of air A by controlling rotational speed ofthe blower 12 by the inverter, or control the supply amount of air Ausing both the inverter and the damper.

The primary fuel controller 86 calculates the supply amount of primaryfuel F1 to be supplied to the can body 10 and controls the supply amountof primary fuel F1 to be supplied to the can body 10 so as to be thecalculated supply amount. Specifically, the primary fuel controller 86calculates a target supply amount of primary fuel F1 based on the supplyamount of air A to the duct 14. The primary fuel controller 86 obtainsinformation on the differential pressure between the upstream side andthe downstream side of the pressure reducing member 12A from the airdifferential pressure sensor 12B shown in FIG. 1, and obtains the supplyamount of air A to the duct 14 from the obtained information on thedifferential pressure. The primary fuel controller 86 may obtaininformation on the supply amount of air A from the air controller 84.Then, in the present embodiment, information indicating a relationshipbetween the supply amount of air A and the supply amount of primary fuelF1 is stored in the storage unit 82. The primary fuel controller 86reads the information indicating the relationship between the supplyamount of air A and the supply amount of primary fuel F1 from thestorage unit 82, and substitutes the obtained supply amount of air Ainto the relationship to calculate the target supply amount of primaryfuel F1. As described above, in the boiler 1, the primary fuelcontroller 86 controls to supply the primary fuel F1 of the targetsupply amount calculated based on the information indicating therelationship between the supply amount of air A and the supply amount ofprimary fuel F1; however, a method for supplying the primary fuel F1 oftarget supply amount is not restricted to this. For example, the boiler1 may be provided with a mechanical governor in the fuel supply line 60,and may be controlled to supply the primary fuel F1 of the target supplyamount by changing an operation amount of governor depending on thedifferential pressure between the upstream side and the downstream sideof the pressure reducing member 12A (that is, the differential pressuredetected by the air differential pressure sensor 12B). That is, theoperation amount of governor may be set in association with the supplyamount (differential pressure) of air A, and the governor may beoperated depending on the supply amount of air A to supply the primaryfuel F1 of the target supply amount. Note that the target supply amountof primary fuel F1 is set such that ratio of oxygen contained in thecombustion gas after the combustion of the primary fuel F1 is, forexample, 6% or more and 10% or less, preferably about 8%. The ratio ofoxygen contained in the combustion gas refers to the ratio of the amountof oxygen contained in the combustion gas to a total amount ofcombustion gas. The combustion gas after the combustion of the primaryfuel F1 refers to the combustion gas in a state in which the combustionreaction of the primary fuel F1 is completed, and it can be said that itrefers to the combustion gas in the state in which the combustionreaction of the primary fuel F1 is completed and the secondary fuel F2is not supplied. Since the combustion reaction may continue in a verysmall amount even in the above-described “combustion gas in the state inwhich the combustion reaction is completed”, “completion of thecombustion reaction” may not mean 100% completion of the combustionreaction, that is, complete combustion. The target supply amount ofprimary fuel F1 can also be rephrased as being set such that an airratio in the mixed gas F1A is a predetermined value. In this case, theair ratio is preferably, for example, 1.4 or more and 2.0 or less. Thetarget supply amount of primary fuel F1 is not limited to the abovedescription and may be calculated by any method.

The primary fuel controller 86 controls the primary fuel supply unit 22so that the primary fuel F1 of the target supply amount thus calculatedis supplied. Specifically, the primary fuel controller 86 controls theopening degree of the primary fuel adjusting valve 62 to supply theprimary fuel F1 for the calculated supply amount to the can body 10. Inthe present embodiment, the primary fuel controller 86 may obtaininformation on the differential pressure of the primary fuel F1 betweenthe upstream side and the downstream side of the pressure reducingmember 64 from the fuel differential pressure sensor 66, and may obtainthe amount of primary fuel F1 actually supplied to the can body 10 fromthe information on the differential pressure. In this case, the primaryfuel controller 86 adjusts the opening degree of the primary fueladjusting valve 62 so that the amount of primary fuel F1 actuallysupplied is the target supply amount. The primary fuel controller 86 isnot limited to adjusting the opening degree of the primary fueladjusting valve 62 when controlling the supply amount of primary fuelF1. For example, the primary fuel controller 86 may control the supplyamount of primary fuel F1 by means of the mechanical governor providedin the fuel supply line 60, or may control the supply amount of primaryfuel F1 using both the governor and the primary fuel adjusting valve 62.

The secondary fuel controller 88 calculates the supply amount ofsecondary fuel F2 to be supplied to the can body 10, and controls thesupply amount of secondary fuel F2 to be supplied to the can body 10 soas to be the calculated supply amount. Specifically, the secondary fuelcontroller 88 obtains the information on the differential pressure ofthe primary fuel F1 between the upstream side and the downstream side ofthe pressure reducing member 64 from the fuel differential pressuresensor 66, and obtains the supply amount of primary fuel F1 to the canbody 10 from the information on the obtained differential pressure. Thesecondary fuel controller 88 may obtain information on the supply amountof primary fuel F1 from the primary fuel controller 86. Then, in thepresent embodiment, information indicating the relationship between thesupply amount of primary fuel F1 and the supply amount of secondary fuelF2 is stored in the storage unit 82. The secondary fuel controller 88reads the information indicating the relationship between the supplyamount of primary fuel F1 and the supply amount of secondary fuel F2from the storage unit 82, and substitutes the obtained supply amount ofprimary fuel F1 in the relationship, to calculate the target supplyamount of secondary fuel F2. The target supply amount of secondary fuelF2 is set such that the ratio of oxygen contained in the combustion gasafter the combustion of the secondary fuel F2 is, for example, 2% ormore and 6% or less, preferably about 4%. The combustion gas after thecombustion of the secondary fuel F2 refers to the combustion gas in astate in which the combustion reaction of the secondary fuel F2 iscompleted. Furthermore, it may be rephrased that the combustion gasafter the combustion of the secondary fuel F2 is the combustion gas in astate in which the combustion reaction of both the primary fuel F1 andthe secondary fuel F2 is completed, and may be rephrased that it is thecombustion gas (exhaust gas) discharged from the can body 10. Further,it can be rephrased that the target supply amount of secondary fuel F2is set such that the air ratio in the case of summing the mixed gas F1Aand the mixed gas F2A is a predetermined value. In this case, the airratio is preferably, for example, 1.1 or more and 1.4 or less. Thetarget supply amount of secondary fuel F2 is not limited to the abovedescription and may be calculated by any method.

Thus, the target supply amount of secondary fuel F2 is set depending onthe supply amount of primary fuel F1. When the supply amount of primaryfuel F1 is adjusted by the information on the differential pressure fromthe fuel differential pressure sensor 66, the supply of the secondaryfuel F2 is also appropriately performed by responding to the supplyamount of primary fuel F1. However, a method for calculating the targetsupply amount of secondary fuel F2 is not limited to this. For example,an oxygen concentration sensor may be provided in the exhaust stack 18,and the secondary fuel controller 88 may set the target supply amount ofsecondary fuel F2 such that the ratio of oxygen contained in thecombustion gas after combustion of the secondary fuel F2 is apredetermined ratio in response to a detection result of oxygenconcentration contained in the combustion gas in the exhaust stack 18 bythe oxygen concentration sensor.

The secondary fuel controller 88 controls the secondary fuel supply unit24 so that the secondary fuel F2 of the target supply amount thuscalculated is supplied. Specifically, the secondary fuel controller 88controls the opening degree of the secondary fuel adjusting valve 72, tosupply the can body 10 with the secondary fuel F2 for the calculatedsupply amount. The secondary fuel controller 88 is not limited toadjusting the opening degree of the secondary fuel adjusting valve 72when controlling the supply amount of secondary fuel F2. For example,the secondary fuel controller 88 may control the supply amount ofsecondary fuel F2 by means of the mechanical governor provided in thesecondary fuel supply line 70, or may control the supply amount ofsecondary fuel F2 using both the governor and the secondary fueladjusting valve 72.

Note that it can be said that the supply amount of secondary fuel F2 isan amount capable of consuming about 8% of oxygen contained in thecombustion gas to about 4% by combustion, and it is proportional to thesupply amount of primary fuel F1. That is, the secondary fuel controller88 increases the supply amount of secondary fuel F2 in proportion to anincrease of the supply amount of primary fuel F1.

The fluid controller 90 controls the flow rate adjusting unit 28 tochange the amount of cooling fluid G0 supplied into the can body 10depending on the supply amount of secondary fuel F2. More specifically,the fluid controller 90 calculates the supply amount of cooling fluid G0to be supplied to the can body 10 and controls the supply amount ofcooling fluid G0 to be supplied to the can body 10 so as to be thecalculated supply amount. The fluid controller 90 controls the flow rateadjusting unit 28 to supply the can body 10 with the cooling fluid G0for the calculated supply amount.

The fluid controller 90 calculates the amount of cooling fluid G0 to besupplied into the can body 10 based on the supply amount of secondaryfuel F2. Specifically, the fluid controller 90 calculates the supplyamount of cooling fluid G0 such that the temperature of thepredetermined space in the can body 10 is within a predeterminedtemperature range. The predetermined temperature range is 800° C. ormore and 1200° C. or less, but more preferably 1000° C. or more and1200° C. or less. Further, as described above, since the predeterminedspace is a space in which the unburned portion of the primary fuel F1and the secondary fuel F2, or the secondary fuel F2 is burned, it can besaid that the fluid controller 90 controls the flow rate adjusting unit28 to adjust a flow rate of the cooling fluid G0 such that thecombustion temperature by the primary fuel F1 and the secondary fuel F2is within the predetermined temperature range. In other words, it can besaid that the fluid controller 90 supplies the cooling fluid G0 for theflow rate capable of setting the combustion temperature in thepredetermined space within the predetermined temperature range. Forexample, in the present embodiment, information indicating therelationship between the supply amount of secondary fuel F2 and thesupply amount of cooling fluid G0 such that the temperature of thepredetermined space in the can body 10 is within the predeterminedtemperature range is stored in the storage unit 82. The air controller84 reads the information from the storage unit 82, and substitutes thesupply amount of secondary fuel F2 into the relationship to calculatethe supply amount of cooling fluid G0. Since the supply position of thesecondary fuel F2 (connection position of the secondary fuel supply line74) is provided at a position in which the temperature of the combustiongas is 800° C. or higher, it can also be said that the fluid controller90 controls the supply amount of cooling fluid G0 so that a temperatureat a predetermined position is 1200° C. or less.

As described above, the fluid controller 90 calculates the supply amountof cooling fluid G0 to the can body 10 based on the supply amount ofsecondary fuel F2. However, a method for calculating the supply amountof cooling fluid G0 is not limited to this. For example, the fluidcontroller 90 may calculate the supply amount of cooling fluid G0 to thecan body 10 based on the supply amount of primary fuel F1, may calculatethe supply amount of cooling fluid G0 to the can body 10 based on thesupply amount of air A to the duct 14, or may calculate the supplyamount of cooling fluid G0 to the can body 10 based on the combustionstage instructed to the boiler 1. In this case, for example, the fluidcontroller 90 stores in the storage unit 82, information indicating arelationship between the supply amount of primary fuel F1 and the supplyamount of cooling fluid G0, information indicating a relationshipbetween the supply amount of air A and the supply amount of coolingfluid G0, or information indicating a relationship between thecombustion stage and the supply amount of cooling fluid G0. The fluidcontroller 90 reads the information from the storage unit 82 andsubstitutes the supply amount of primary fuel F1, the supply amount ofair A, or the combustion stage into the relationship to calculate thesupply amount of cooling fluid G0. The boiler 1 may be provided with anitrogen oxide concentration sensor in the exhaust stack 18, and thesupply amount of cooling fluid G0 to the can body 10 may be calculatedbased on a detection result of the nitrogen oxide concentration sensor.In this case, the fluid controller 90 obtains information on nitrogenoxide concentration in the cooling fluid G0 in the exhaust stack 18detected by the nitrogen oxide concentration sensor. Then, the fluidcontroller 90 calculates the flow rate of the cooling fluid G0 from theobtained nitrogen oxide concentration such that the content of thenitrogen oxide is within a predetermined range. The fluid controller 90controls the flow rate adjusting unit 28 to supply the can body 10 withthe cooling fluid G0 for the calculated flow rate.

As described above, the cooling fluid G0 can reduce the temperature ofthe combustion gas, and the temperature can be further reduced as thesupply amount of cooling fluid G0 increases. Here, the combustiontemperature in the can body 10 depends on the combustion amount, thatis, the supply amounts of the primary fuel F1, the secondary fuel F2,and the air A; however, the supply amount of secondary fuel F2 iscalculated based on the supply amount of primary fuel F1, and the supplyamount of primary fuel F1 is calculated based on the supply amount ofair A. That is, the supply amounts of the primary fuel F1, the secondaryfuel F2, and the air A are related to each other. Therefore, it can besaid that the combustion temperature in the can body 10 depends on thesupply amount of primary fuel F1. Therefore, it can be said that thefluid controller 90 changes the supply amount of cooling fluid G0depending on the supply amount of primary fuel F1 in order to maintainthe temperature in the predetermined space in the predeterminedtemperature range. That is, the fluid controller 90 increases the amountof cooling fluid G0 to be supplied as the supply amount of primary fuelF1 increases, and reduces the amount of cooling fluid G0 to be suppliedas the supply amount of primary fuel F1 is reduced.

The supply amount of cooling fluid G0 will be further described. FIG. 4is a graph showing a relationship between the amount of primary fuel,the amount of secondary fuel, and the amount of cooling fluid. In FIG.4, a horizontal axis is the supply amount of primary fuel F1. A linesegment L1 indicates the relationship between the supply amount ofprimary fuel F1 and the supply amount of secondary fuel F2. As indicatedby the line segment L1, the secondary fuel controller 88 linearlyincreases the supply amount of secondary fuel F2 in response to a linearincrease of the supply amount of primary fuel F1. That is, a rate atwhich the flow rate of the secondary fuel F2 increases when the supplyamount of primary fuel F1 increases is substantially constant. A linesegment L2 indicates a relationship between the supply amount of primaryfuel F1 and the supply amount of cooling fluid G0. As indicated by theline segment L2, when the supply amount of primary fuel F1 linearlyincreases, the fluid controller 90 increases the supply amount ofsecondary fuel F2 in the form of a quadratic curve convex downward. Thatis, the fluid controller 90 controls the supply amount of cooling fluidG0 such that the rate increases at which the supply amount of coolingfluid G0 increases when the supply amount of primary fuel F1 increases,as the supply amount of primary fuel F1 increases. In the case of a highcombustion state in which the supply amount of primary fuel F1 is large,a rate of increase in temperature of the can body 10 may be high. Thefluid controller 90 can preferably suppress temperature rise in the highcombustion state by increasing the supply amount of cooling fluid G0 inthe high combustion state as in the line segment L2.

The control device 30 is structured as described above. Next, a flow ofmethod of supplying fuel and the like by the control device 30 will bedescribed based on a flowchart. FIG. 5 is a flowchart illustrating acontrol flow of the control unit according to the first embodiment. Asshown in FIG. 5, first, the control unit 80 calculates the supply amountof air A by the air controller 84, and controls an air blowing amount sothat the calculated amount of air A is supplied (Step S10). Then, thecontrol unit 80 calculates the supply amount of primary fuel F1 based onthe supply amount of air A by the primary fuel controller 86, andcontrols the primary fuel adjusting valve 62 so that the calculatedamount of primary fuel F1 is supplied (Step S12). Then, the control unit80 calculates the supply amount of secondary fuel F2 based on the supplyamount of primary fuel F1 by the secondary fuel controller 88, andcontrols the secondary fuel adjusting valve 72 so that the calculatedamount of secondary fuel F2 is supplied (Step S14). Then, the controlunit 80 calculates the supply amount of cooling fluid G0 such that thepredetermined space in the can body 10 has the predetermined temperaturerange (for example, 800° C. or more and 1200° C. or less) based on thesupply amount of secondary fuel F2 by the fluid controller 90, andcontrols the flow rate adjusting unit 28 so that the calculated amountof cooling fluid G0 is supplied (Step S16). Thus, the present process iscompleted by supplying the primary fuel F1, the air A, the secondaryfuel F2, and the cooling fluid G0.

In description of FIG. 5, detection of the combustion stage, supplycontrol of the air A based on the combustion stage, supply control ofthe primary fuel F1 based on the supply amount of air A, supply controlof the secondary fuel F2 based on the supply amount of primary fuel F1,and supply control of the cooling fluid G0 based on the supply amount ofsecondary fuel F2 are performed in this order. However, each control isnot limited to being performed in this order. For example, the controldevice 30 may store in the storage unit 82 in advance, the information(table) indicating the relationship between the combustion stage and thesupply amount of air A, information indicating a relationship betweenthe combustion stage and the supply amount of primary fuel F1,information indicating a relationship between the combustion stage andthe supply amount of secondary fuel F2, and information indicating arelationship between the combustion stage and the supply amount ofcooling fluid G0. In this case, the controller 30 reads the information,calculates the supply amount of air A, the supply amount of primary fuelF1, the supply amount of secondary fuel F2, and the supply amount ofcooling fluid G0 from the combustion stage instructed to the boiler 1,and supplies the air A, the primary fuel F1, the secondary fuel F2, andthe cooling fluid G0 so that they are the calculated supply amounts.After performing the supply control in this manner, the control device30 may finely adjust the supply amounts by performing the supply controlof the primary fuel F1 based on the supply amount of air A, the supplycontrol of the secondary fuel F2 based on the supply amount of primaryfuel F1, and the supply control of the cooling fluid G0 based on thesupply amount of secondary fuel F2 as described above.

As described above, the boiler 1 according to the present embodiment hasthe can body 10 having the water pipes 51, 52, 53, the burner 16, thesecondary fuel supply unit 24, the cooling line 26, and the flow rateadjusting unit 28, and the control unit 80. The burner 16 is connectedto the can body 10 and supplies the primary fuel F1 and the air A intothe can body 10. The secondary fuel supply unit 24 supplies thesecondary fuel F2 into the can body 10 downstream of the burner 16 inthe flow direction of the combustion gas. The cooling line 26 introducesthe cooling fluid G0 for reducing the temperature in the predeterminedspace in the can body 10 downstream of the burner 16 in the flowdirection of the combustion gas. The flow rate adjusting unit 28 isprovided in the cooling line 26 and can adjust the flow rate of thecooling fluid G0 to be introduced into the can body 10 from the coolingline 26. The control unit 80 controls the flow rate adjusting unit 28and supplies the flow rate of the cooling fluid G0 to be introduced intothe can body 10 such that the temperature of the predetermined space is800° C. or more and 1200° C. or less.

The boiler 1 according to the present embodiment suppresses thegeneration of CO by setting the temperature of the predetermined spacein the can body 10 to 800° C. or higher. Further, the boiler 1 reducesNOx by setting the temperature of the predetermined space in the canbody 10 to 1200° C. or less by the cooling fluid G0. Here, for example,when the boiler 1 having a plurality of combustion stages performs lowcombustion as described above, if the combustion stage is shifted toincrease the amount of combustion from the required load, since thetemperature rise is large, it may be difficult to maintain thetemperature in the predetermined space at 800° C. to 1200° C. On theother hand, the boiler 1 according to the present embodiment can controlthe temperature in the predetermined space to 800° C. to 1200° C. bycooling with the cooling fluid G0. Therefore, according to the boiler 1,even if the combustion stage is changed, NOx and CO can be appropriatelyreduced by controlling the flow rate adjusting unit 28 to adjust theflow rate of the cooling fluid G0. Furthermore, the boiler 1 accordingto the present embodiment may control the supply amount of the coolingfluid G0 depending on the supply amount of primary fuel F1, thesecondary fuel F2, or the air A. In this case, for example, the boiler 1can suppress that the supply amount of the cooling fluid G0 is excessiveand the temperature is too low when the supply amount of primary fuelF1, the secondary fuel F2 or the air A is small, or suppress that thesupply amount of the cooling fluid G0 is insufficient and thetemperature is too high when the supply amount of primary fuel F1, thesecondary fuel F2 or the air A is large.

The control unit 80 controls the flow rate adjusting unit 28 such thatthe rate increases at which the flow rate of the cooling fluid G0increases when the supply amount of primary fuel F1 increases, as thesupply amount of primary fuel F1 increases. The boiler 1 canappropriately suppress the temperature rise in the high combustion stateby thus increasing the supply amount of the cooling fluid G0 in the highcombustion state.

The secondary fuel supply unit 24 (secondary fuel supply line 70) isconnected to the cooling line 26, and supplies the secondary fuel F2into the can body 10 in a state of being mixed with the cooling fluidG0. Since the boiler 1 supplies the secondary fuel F2 into the can body10 in a state of being mixed with the cooling fluid G0, it is possibleto restrain the temperature from being excessively raised by the coolingfluid G0 while being suitably burned in two stages by the secondary fuelF2.

However, the cooling line 26 may not be connected to the secondary fuelsupply unit 24, and the secondary fuel F2 and the cooling fluid G0 maybe separately supplied into the can body 10 without being mixed. In thiscase, the cooling line 26 is preferably connected to the can body 10upstream of the secondary fuel supply unit 24 in the flow of thecombustion gas. That is, in this case, the cooling line 26 is connectedto a position between the ignition unit (not shown) and the secondaryfuel supply unit 24 (secondary fuel supply line 74). Therefore, in thiscase, it can be said that the secondary fuel supply unit 24 supplies thesecondary fuel F2 into the can body 10 downstream of the cooling line 26in the flow direction of the combustion gas. Thus, by supplying thesecondary fuel F2 to the downstream side of the cooling fluid G0, thecombustion gas can be cooled, and the reaction with the secondary fuelF2 can be slowed to suppress the temperature rise. Further, by supplyingthe secondary fuel F2 to the downstream side of the cooling fluid G0,that is, by supplying the cooling fluid G0 to the upstream side of thesecondary fuel F2, it is possible to block the flame of the combustiongas by the primary fuel F1 from going downstream by the cooling fluidG0. Thus, it is possible to restrain the flame of the combustion gasfrom coming into contact with the secondary fuel F2, and to suppress thetemperature rise due to the secondary fuel F2 burning with the flame.

The flow rate adjusting unit 28 is a fan for supplying the exhaust gasdischarged from inside the can body 10 into the cooling line 26. Thecooling line 26 introduces the exhaust gas supplied from the flow rateadjusting unit 28 into the can body 10 as the cooling fluid G0. Theboiler 1 can suitably cool the inside of the can body 10 by using theexhaust gas as the cooling fluid G0. Further, by introducing the exhaustgas into the can body 10, it also functions as EGR (Exhaust GasRecirculation), and NOx can be suitably reduced. Furthermore, by settingthe flow rate adjusting unit 28 as a fan, it can suitably take in theexhaust gas as the cooling fluid G0, and suitably control an intakeamount of the exhaust gas.

However, the cooling fluid G0 is not limited to the exhaust gas, and maybe any fluid as long as it is a fluid capable of reducing thetemperature in the can body 10 raised by the combustion gas. Morespecifically, it is preferable that the cooling fluid G0 be a fluid thatis not burned by the combustion gas, and be a fluid that is cooler thanthe combustion gas in the can body 10. The cooling fluid G0 ispreferably a gas, but may be a liquid. Examples of the cooling fluid G0other than the exhaust gas include steam, water, and an inert gas. Thatis, the cooling line 26 may introduce at least one or more of theexhaust gas discharged from inside the can body 10, the steam, thewater, and the inert gas into the can body 10 as the cooling fluid G0.By using such a cooling fluid G0, the temperature rise in the can body10 can be suitably suppressed. Note that examples of the inert gasinclude nitrogen, carbon dioxide, and argon.

Second Embodiment

Next, a second embodiment will be described. A boiler 1 a according tothe second embodiment shows an example in which the steam is used as thecooling fluid. Descriptions of portions of the second embodiment havingthe same structure as that in the first embodiment will be omitted.

FIG. 6 is a schematic cross-sectional view of the boiler according tothe second embodiment. As shown in FIG. 6, the boiler 1 a according tothe second embodiment has a cooling line 26 a and a flow rate adjustingunit 28 a. One end of the cooling line 26 a is connected to the upperheader 42 or a steam header (not shown), and the other end thereof isconnected to the secondary fuel supply line 70 through an ejector 76 a.The cooling line 26 a is supplied with the steam from the upper header42 as a cooling fluid G0 a. The cooling fluid G0 a flowing through thecooling line 26 a is supplied to the secondary fuel supply line 74through the secondary fuel supply line 70 and introduced into the canbody 10 in a state of being mixed with the secondary fuel F2. The flowrate adjusting unit 28 a is an on-off valve which is controlled to beopened or closed by the fluid controller 90 of the control device 30.The flow rate adjusting unit 28 a adjusts the supply amount of thecooling fluid G0 a into the can body 10 by being controlled to be openedand closed. Since the cooling fluid G0 a is the steam, the cooling line26 a may not be provided with the fan for taking in the cooling fluid G0a.

FIG. 7 is a schematic view of the ejector according to the secondembodiment. As shown in FIG. 7, the ejector 76 a has an inner cylinder76 a 1 and an outer cylinder 76 a 2. The inner cylinder 76 a 1 isprovided inside the outer cylinder 76 a 2. One end of the inner cylinder76 a 1 is connected to the cooling line 26 a, and the other end thereofis tapered. One end of the outer cylinder 76 a 2 is connected to thesecondary fuel supply line 70, and the other end thereof is connected tothe secondary fuel supply line 74. The cooling fluid G0 a supplied fromthe cooling line 26 a to the inner cylinder 76 a 1 is injected from theinner cylinder 76 a 1 into the outer cylinder 76 a 2, and flows from theouter cylinder 76 a 2 to the secondary fuel supply line 74. On the otherhand, the secondary fuel F2 is drawn into the outer cylinder 76 a 2 frominside the secondary fuel supply line 70 by injection of the coolingfluid G0 a into the outer cylinder 76 a 2. The secondary fuel F2 drawninto the outer cylinder 76 a 2 flows from the outer cylinder 76 a 2 tothe secondary fuel supply line 74, and is mixed with the cooling fluidG0 a.

As in the boiler 1 a according to the second embodiment, even if thesteam is used as the cooling fluid G0 a, NOx and CO can be appropriatelyreduced as in the first embodiment. The boiler 1 a according to thesecond embodiment has the ejector 76 a connected to the cooling line 26a through which the cooling fluid G0 a flows and the secondary fuelsupply line 70 through which the secondary fuel F2 flows. The ejector 76a injects the cooling fluid G0 a from the cooling line 26 a into aninside thereof to draw the secondary fuel F2 from the secondary fuelsupply line 70, and supplies the secondary fuel F2 and the cooling fluidG0 a to the secondary fuel supply line 74 connected thereto. Thus, theboiler 1 a has the ejector 76 a for taking in the secondary fuel F2using vapor pressure of the cooling fluid G0 a, so that the secondaryfuel F2 can be appropriately taken therein to be mixed with the coolingfluid G0 a even when a supply pressure of the secondary fuel F2 is low.In addition, it is also possible to perform secondary combustion, forexample, using a low dryness steam.

1. A boiler comprising: a can body having water pipe; a burner connectedto the can body and for supplying primary fuel and air into the canbody; a secondary fuel supply unit for supplying secondary fuel into thecan body downstream of the burner in a flow direction of combustion gas;a cooling line for introducing a cooling fluid for reducing temperatureof a predetermined space in the can body downstream of the burner in theflow direction of the combustion gas; a flow rate adjusting unitprovided in the cooling line and capable of adjusting a flow rate of thecooling fluid introduced into the can body from the cooling line; and acontrol unit for controlling the flow rate adjusting unit to control theflow rate of the cooling fluid introduced into the can body such thatthe temperature of the predetermined space is 800° C. or more and 1200°C. or less.
 2. The boiler according to claim 1, wherein the control unitcontrols the flow rate adjusting unit such that a rate increases atwhich the flow rate of the cooling fluid increases when a supply amountof the primary fuel increases, as the supply amount of the primary fuelincreases.
 3. The boiler according to claim 1, wherein the secondaryfuel supply unit is connected to the cooling line, and supplies thesecondary fuel into the can body in a state of being mixed with thecooling fluid.
 4. The boiler according to claim 1, wherein the secondaryfuel supply unit supplies the secondary fuel into the can bodydownstream of the cooling line in the flow direction of the combustiongas.
 5. The boiler according to claim 1, wherein the cooling lineintroduces at least one or more of exhaust gas discharged from the canbody, steam, water, and inert gas into the can body as the coolingfluid.
 6. The boiler according to claim 3, wherein the flow rateadjusting unit is a fan for supplying the exhaust gas discharged fromthe can body into the cooling line, and the cooling line introduces theexhaust gas supplied from the flow rate adjusting unit into the can bodyas the cooling fluid.
 7. (canceled)
 8. The boiler according to claim 2,wherein the secondary fuel supply unit is connected to the cooling line,and supplies the secondary fuel into the can body in a state of beingmixed with the cooling fluid.
 9. The boiler according to claim 2,wherein the secondary fuel supply unit supplies the secondary fuel intothe can body downstream of the cooling line in the flow direction of thecombustion gas.
 10. The boiler according to claim 3, wherein thesecondary fuel supply unit supplies the secondary fuel into the can bodydownstream of the cooling line in the flow direction of the combustiongas.
 11. The boiler according to claim 2, wherein the cooling lineintroduces at least one or more of exhaust gas discharged from the canbody, steam, water, and inert gas into the can body as the coolingfluid.
 12. The boiler according to claim 3, wherein the cooling lineintroduces at least one or more of exhaust gas discharged from the canbody, steam, water, and inert gas into the can body as the coolingfluid.
 13. The boiler according to claim 4, wherein the cooling lineintroduces at least one or more of exhaust gas discharged from the canbody, steam, water, and inert gas into the can body as the coolingfluid.